254 Steel Girder Bridges

Steel has higher strength, ductility, and toughness than many other structural materials such as concrete or wood, and thus makes a vital material for bridge structures. In addition to the conventional steel, there are many types of high-performance steel (HPS) developed recently for the bridge application.

Steel with low yield ratio (SN, SA440)

nu: Tensile strength oy Yield point

£u: Uniform elongation

£y: Yield strain

Elongation y y u u

Elongation

Steel with no specification for yield ratio nu: Tensile strength oy Yield point

HPS can be defined as having an optimized balance of their properties, such as strength, weldability, toughness, ductility, corrosion resistance, and formability, to give maximum performance in bridge structures while remaining cost-effective (Mistry 2002). The two main differences compared to conventional weathering steels are improved weldability and toughness. Other properties such as corrosion resistance and ductility will be essentially the same. There are two HPS grades, ASTM A 709 HPS 70W and HPS 50W, available for bridge structures in the United States. There are 91 HPS bridges built in the United States so far (FHWA 2003). HPS has very high strength, with constant yield point, narrow range of yield point, low yield ratio, excellent ductility, and ultrathickness. Figure 25.12 shows the performance mechanism of the steel with low yield ratio that provides higher ductility at plastic hinges.

Girder bridges are structurally the simplest and the most common. They consist of a floor slab, girders, and bearings, which support and transmit gravity loads to the substructure. Girders resist bending moments and shear forces and are used to span short distances. Steel girders are classified as plate and box girders.

Figure 25.13 shows the structural composition of plate and box girder bridges and the load transfer path. In plate girder bridges, the live load is directly supported by the slab and then by the main girders. In box girder bridges, the forces are taken first by the slab, then supported by the stringers and floor beams in conjunction with the main box girders, and finally taken to the substructure and foundation through the bearings.

Girders are classified as noncomposite or composite, that is, whether the steel girders act in tandem with the concrete slab (using shear connectors) or not. Since composite girders make use of best properties of both steel and concrete, they are often the rational and economic choice. Less frequently noncomposite I-shapes are used for short-span noncomposite bridges.

25.4.2 Plate Girder (Noncomposite)

The plate girder is the most economical shape designed to resist bending and shear and the moment of inertia is the greatest for a relatively low weight per unit length. Figure 25.14 shows a plan of a typical plate girder bridge with four main girders spanning 30 m and a width of 8.5 m.

The gravity loads are supported by several main plate girders, each manufactured by welding three plates: top and bottom flanges and a web. Figure 25.15 shows a piece of plate girder and its fabrication

process. The web and the flanges are cut from the steel plate and welded. The piece is fabricated in shop and transported to the construction site for erection.

The design procedure for plate girders, primarily the sizing of the three plates, is discussed as follows:

1. Web height. The web height is the fundamental design factor affecting the weight and cost of the bridge. If the height is too small, the flanges need to be large and the dead weight increases. The height (D) is determined empirically by dividing the span length (L) by a "reasonable" factor. Common ratios are D/L — 1g to 20 for highway bridges and a little smaller for railway bridges. The web height also influences the flexural stiffness of bridge. Greater heights generally produce greater stiffness. However, if the height is too large, the web may becomes unstable and must have its thickness supplemented or stiffeners added. These measures increase the weight and the cost. In addition, plate girders with excessively deep web and small flanges are vulnerable to buckle laterally.

2. Web thickness. The web primarily resists shear forces that are not usually significant when the web height is properly designed. The shear force is generally assumed to be distributed uniformly across the web instead of using the exact equation of beam theory. The web thickness (t) is determined such that thinner is better as long as buckling is prevented. Since the web does not contribute much to the bending resistance, thin webs are most economical but the possibility of buckling increases. Therefore, a noncompact web thickness is usually selected and stiffened by transverse (vertical) and longitudinal (horizontal) stiffeners. It is not primarily strength but rather stiffness that controls the design of webs.